This invention relates to a readout circuit for a magnetic storage and readout device, or more in particular to a readout circuit suitable for obtaining equalization characteristics which are optimum for reducing a pattern peak shift.
Generally, a digital magnetic recording device such as a magnetic disk drive posed the problem of a pattern peak shift which is a cause of data reproducing error. FIG. 26 is a diagram showing an example of a pattern peak shift. This diagram indicates the manner in which a pattern peak shift occurs when two data including B.sub.1 and B.sub.2 are recorded. Assume that the waveform presented by W.sub.1 and W.sub.2 is corresponding to a reproduced signal from an isolated data B.sub.1 and B.sub.2, respectively. A readout waveform W is obtained when two data of B.sub.1 and B.sub.2 are recorded and is equal to superposition of W.sub.1 and W.sub.2 which interfere with each other, the result being that the amplitude thereof is reduced and the peak positions are displaced as shown by P.sub.1 and P.sub.2.
As seen from FIG. 26, pattern peak shift would not occur if the isolated readout waveform W.sub.1 or W.sub.2 has narrow spread skirts so as not to interfer with adjacent waveforms. Conventional devices have an equalizer for equalizing an isolated pulse waveform into a sharp wave form by cutting off the skirt thereof as disclosed in IEEE Trans. Magnetics, MAG-12, (1976) pp. 746 to 748 in order to reduce pattern peak shift.
In the case of a magnetic disk drive the head flying height on the outer radius of the medium is larger than that on the inner radius thereof, resulting in different isolated pulse waveforms. As diclosed in JP-A-51-167811, JP-A-61-139980 and JP-A-61-208606, equilizer coefficients, predetermined corresponding to track position, are selected or equalizer coefficients read out from an read only memory set in an equilizer in response to a head radius position signal. Such a system, in which the same equalizer coefficient is always set for a given track position, has the disadvantage mentioned below. Specifically, if the recording and readout characteristics undergo a change due to a change of head flying height or medium characteristics which are caused by production variations or a change on standing, it is necessary to also change the optimized equalization characteristics. Since the same equalization coefficient is set all the time, however, the optimized equalization characteristics cannot be obtained, and consequently the compensation is insufficient, thereby cause a reading error. On the other hand, a method has been suggested in which, as disclosed in JP-A-58-50612 and JP-A-60 -113363, the pulse width at 50% amplitude point of an isolated pulse waveform (PW.sub.50) of relative timing difference between the data pulse and the detection window is detected for continuous control of an equalizer. According to this method, ideal automatic equalization is possible if an isolated pulse waveform can be approximated by Lorentzian as in the case where a waveform is read out with a head having an ordinary ferrite head. It is, however, impossible to obtain an optimally equalized waveform in the case where the isolated waveform sharply expands in the neighbourhood of the foot of a peak thereof as seen from IEEE Trans. Magnetics, MAG-22, (1986) pp. 1209 to 1211 or includes an understood in such a case as with a thin film head.